WIP: Activate Credential package

[josephlr]

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This is a rough draft of what an activate package would look like in go-tpm-tools

Ideally, it would provide two things:

• A set of functions to run on the server side to verify requests and send back encrypted blobs
• A set of functions to run on the client side that perform the necessary tpm2 commands to activate a credential.

Note that this is rebased on https://github.com/google/go-tpm-tools/pull/3, so ignore those commits

DO NOT SUBMIT: need feedback, more testing, and documentation